1. Technical Field
The present disclosure relates to a wide-band low-noise amplifier (LNA) and, more particularly, to a wide-band radio frequency (RF) LNA with a controllable gain.
2. Discussion of the Related Art
Typically, wide-band wireless communication receivers use a high performance front-end amplifier, which is referred to as a low-noise amplifier. The low-noise amplifier should have excellent noise and linearity characteristics in a wide frequency range and for a wide range of input signal power.
It is very difficult to implement a front-end gain controllable amplifier that combines low noise and high linearity over a wide frequency range. In a front-end amplifier, the trade off between noise and linearity can be managed by controlling the gain according to the magnitude of the input signal. When the input signal is weak, the low-noise amplifier (LNA) requires a high gain and low noise performance. In contrast, when the input signal is strong, the LNA requires a relatively lower gain and high linearity performance.
FIGS. 1A and 1B are circuit diagrams of conventional amplifiers. In FIG. 1A, an amplifier includes an amplifying transistor T1 and a load RD. The gain Av, input impedance Rin, and noise factor F of the amplifier of FIG. 1A can be expressed by Equation 1.
                              Av          =                                    g              m                        ⁢                          R              D                                      ⁢                                  ⁢                  Rin          =                      1                          g              m                                      ⁢                                  ⁢                              F            =                          1              +                              γ                                                      g                    m                                    ⁢                                      R                    S                                                                                ,                                    [                  Equation          ⁢                                          ⁢          1                ]            where “gm” is the transconductance of the amplifying transistor T1, “RS” is the source resistance of an input power, and “γ” is a parameter of the amplifying transistor T1.
In FIG. 1B, an amplifier includes the amplifying transistor T1, the first load RD and a second load RF. The gain Av, input impedance Rin, and noise factor F of the amplifier of FIG. 1B can be expressed by Equation 2.
                              Av          =                      -                                                            g                  m                                ⁢                                  R                  D                                                            1                +                                                      R                    D                                    /                                      R                    F                                                                                      ⁢                                  ⁢                  Rin          =                                    1                              g                m                                      ⁢                          (                              1                +                                                      R                    F                                                        R                    D                                                              )                                      ⁢                                  ⁢                              F            =                          1              +                              3.6                                                      g                    m                                    ⁢                                      R                    S                                                                                ,                                    [                  Equation          ⁢                                          ⁢          2                ]            where “gm” is the transconductance of the amplifying transistor T1 and “RS” is the source resistance of an input power.
As can be seen from Equations 1 and 2, in a conventional amplifier, a trade-off relationship exists between the gain Av and the input impedance Rin. That is, to control the gain Av, when the transconductance of the amplifying transistor T1 is adjusted, the input impedance Rin is also adjusted.
For a conventional LNA used in a communication receiver, the input impedance needs to match a predetermined value, for example, 75 Ω or 50 Ω. However, as described above, for a conventional amplifier, when the gain is adjusted, the input impedance is also adjusted and the correct impedance matching is not maintained.